1. Field of the Invention
The present invention relates generally to a digital camera, and more particularly, to a method and apparatus for auto-focus control of a digital camera.
2. Description of the Related Art
Recently, digital cameras, in general, and in particular high-pixel digital cameras installed in mobile devices, such as cellular phones, have an auto-focus function for automatically focusing the lens when taking a photograph.
Generally, such an auto-focus control method is one of a phase-difference detection scheme typically employed in Digital Single Lens Reflex (DSLR) cameras, and a contrast detection scheme typically employed in compact digital cameras and cellular phone cameras. The phase-difference detection scheme distributes light, which has passed through a lens, between a viewfinder and an auto-focus (AF) sensor through a mirror, to separate light which is incident on the AF sensor in two directions on a focus surface, to detect both beams of light by means of a line sensor, and to determine if the camera is focusing on a foreground or a background. Such a phase-difference detection scheme requires a lens system and a separate sensor for auto-focusing.
The contrast detection scheme is based on a hill climbing method for finding a position at which an edge value is a maximum while moving a focus lens in given steps within an entire movable range of the focus lens or a part of the movable range. FIG. 1 schematically illustrates the operation of an auto-focus control method using a normal contrast detection scheme. As shown in FIG. 1A, the contrast detection scheme is implemented in such a manner as to trace a change in an edge value, which is obtained by filtering an electrical signal given from an image sensor, according to each position while moving a focus lens at the same intervals, and to move the focus lens to a focus position where the highest focus value is obtained according to a result of the tracing, as shown in FIG. 1B.
In such conventional technologies, it is important to find a position having a maximum value without a large error while controlling an interval between positions between which a lens moves and a movement direction of the lens. However, as a higher accuracy and a high success rate are required, a processing time and the number of positions required for edge value sampling increase. In this case, an auto-focus control time increases, so that an interval between a photographing time point intended by the photographer and a time point at which a photograph is actually taken after auto-focus control may be longer. In particular, since an exposure time of an image sensor becomes longer under a low illumination, the operation time of the auto-focus control further increases.